High quality electrographic process color images require smooth, high quality paper receivers such as clay coated lithographic paper stock. To fuse such process color images on clay coated papers, there is a need for toner binders that display toner melt viscosities that are substantially lower than the melt viscosities associated with typical known toner binders. Reduced viscosity needs to be achieved to obtain the desired fusing properties such as surface gloss and the elimination of light scattering voids within an image while at the same time allowing for low enough input energy or temperature such that the clay coated paper stock does not blister. Blistering is a phenomena where water within the clay coated lithographic paper stock is vaporized during the toner fusing process, causing the paper to form surface protrusions and delaminations. Thus reduced viscosity needs to be achieved to obtain good fusing quality, however, at the same time offset and jamming must be minimized. Offset is a phenomena where the toner deposit fails cohesively, during fusing, some toner stays on the paper and some sticks to the fusing roller, from which it can offset onto the next piece of paper. Resistance to offset is normally associated with high melt cohesive strength or high melt elasticity of the toner, such as is achieved by high molecular weight or crosslinked toner binder polymers. Polyester toner binders represent a useful class of materials which are capable of being tailored to offer resistance to offset. It is an object of the present invention to offer a way of substantially reducing the melt viscosity of polyester toners in order to produce high quality fused process color images at high speed on clay coated lithographic paper without blistering, while at the same time maximizing the resistance to offset. This improvement is achieved via polyblends of an amorphous polyester toner binder with a high T.sub.g, very low molecular weight epoxy resin.
Polyblends of a polyester resin and an epoxy resin are taught in U.S. Pat. No. 4,693,952. The polyester resins have softening points of 110.degree. to 150.degree. C. and the epoxy resins have T.sub.m 's between about 60.degree. and 160.degree. C. and T.sub.g 's of at least about 55.degree. C. The polyblends do not display a substantially reduced melt viscosity compared to the melt viscosity of polyesters used alone.
A toner composition is described in U.S. Pat. No. 4,457,998 wherein the resin particles are comprised of an uncrosslinked polymer, that can be a polyester, dispersed in a highly crosslinked polymer that can be an epoxy polymer. The polymer combination does not have a reduced melt viscosity compared to the polyester alone.
European Patent Application No. 0,197,387 discloses another toner binder composition that comprises a lactone-modified epoxy resin, a styrene-butadiene rubber and a polyester resin. The lactone modification is obtained by subjecting all or a portion of the secondary hydroxyl groups of epoxy resins having hydroxyl groups to ring opening polymerization of lactone. The styrene-butadiene rubber is used to achieve improved offset resistance; the polyester resin is used to increase toner "flexibility"; and the lactone-modified epoxy resin is used to increase toner transferability. A two component polyblend with substantially reduced melt viscosity compared to the polyester alone is not achieved.
So far as now known, no one has provided a two component polyblend of a polyester with a low molecular weight epoxy resin that has a melt viscosity that is substantially less than that of the polyester alone.